Genetic modification of insects as pest control part 2

In part 1 of this series, I discussed the history of genetic modification in insects as pest control. We’ve been creating insect GMOs for the purposes of controlling pests for awhile. If you bombard insects with radiation, it can kill rapidly reproducing cells. High doses of radiation can also damage the DNA in quickly reproducing gamete producing cells to the point where it can’t be read, creating severe mutations that stop important proteins from being made. In other words, sperm are produced, but they aren’t healthy. If female flies mate with one of these males, she won’t produce any offspring. If this happens enough on a large scale, the population plummets because females aren’t producing viable offspring. This technique has been used for years in various disciplines from medicine to agriculture. There’s always room for improvement, and this is no exception.

These guys like fruit and long walks on the beach.

Flash forward 50 years past the screwworm elimination program (discussed in part 1) and several other wildly successful programs. Greatly increased international travel allows pests to spread all over the globe.

One pest, the Mediterranean fruit fly (Ceratitis capitata or Medfly for short), has a host range of over 200 plant species. It specializes on crops where just about any damage justifies control. Medfly larvae feed inside the fruit, so they’re really hard to control because there aren’t Bt crops available for everything and you can’t inject conventional pesticides into the fruit. These flies scare us so much that some agroterrorist groups actually claimed to have used them as weapons.

The most bizarre protest of all has been a letter to Los Angeles Mayor Tom Bradley and local newspapers, sent by an ecoterrorist organization calling itself the Breeders, which claimed to be breeding and releasing its own medflies. The organization’s alleged purpose: to render the medfly problem “unmanageable” and Malathion spraying “financially intolerable.”

Don’t worry, the ecoterrorists don’t have the upper hand here. Medfly biology isn’t that much different from the standard model organism Drosophila melanogaster. Thanks to a TON of prior experience with rearing flies (like Drosophila) and the unique characteristics of Medfly biology, we’re able to mess around with Medfly biology and figure stuff out.

For starters, insect genetics are weird… like really weird. Insect larvae are completely different from the adults. The medfly goes from a legless, rasping maggot to a flying, walking adult which feeds on a liquid diet. In fact, you need completely separate keys to identify the two. Accordingly, the two use almost completely different sets of genes. Some are only turned on during the larval stage, and others are only turned on during the adult.

Imagine that you could take a gene for a lethal protein which is only expressed in larvae and engineer it so that the gene only expresses when a specific chemical is present (the gene is off unless the chemical is there). Another option is to have a lethal gene that only expresses when a specific chemical is absent (the gene is on unless the chemical is there). You’ve now got a way to turn expression of that gene on or off. Either way, the chemical isn’t around in the wild… so the larvae thrive in the lab and die in the wild.

This type of conditional gene expression for Medfly larvae was described in 2005 by scientists from the University of Oxford. The lead scientist on this research, Luke Alphey, created a spin off company called Oxitec to develop and commercialize this type of insect control system. The system is called RIDL, Release of Insects carrying a Dominant Lethal.

The system they developed had a dominant lethal gene being expressed in larvae unless a specific chemical, tetracycline, was present. In the diagram below (from the Oxitec website), that gene is for the “tetracycline transactivator” protein or tTA for short. When tetracycline (Tc) is present, it binds to the tTA protein so tTA can’t bind to the genetic regulatory element tetO. Mefly larvae with the RIFL transgene thrive when Tc is present.

If tetracycline (Tc) isn’t present, the protein produced by the tTA gene causes more expression of tTA by binding to tetO. Then, tTa accumulates and does some pretty nasty stuff to the larvae. We’re not quite sure what it does… but it’s lethal. Dead larvae can’t grow into adults and reproduce. Ideally, the majority of larvae will inherit the RIDL transgene which means that the majority of the larvae will die. This system protein works beautifully in the lab, but of course real-world tests need to be performed.

The larvae need high levels of the tTA protein in their systems in order for it to kill them. If the occasional larva is accidentally ingested by a mouse or human no ill effects will occur. Even in mice which have been engineered to express this gene, tTA can’t build up to harmful levels.

The RIDL system is a big improvement over the traditional technique of irradiating insects (as described in part 1) for a number of ways.

First, we don’t have to sterilize the larvae because they’re going to die outside of the lab if they don’t get tetraycline. That means workers don’t have to work around radiation or other mutagens. We also don’t have to worry about an incompetent lab tech or a natural disaster accidentally releasing unsterilized insects into the wild. Even if the tTA system fails and the larvae are able to develop into adults, the tTA protein has a different mode of action from any pesticide so the RIDL gene won’t contribute to pesticide resistance.

Another way this is an improvement is its subtlety. Insects compete with each other all the time in the wild even within the same species. If you simply release insects that can’t reproduce, you’re opening up a bunch of resources for those who can. There’s less competition between larvae, so those which are lucky enough to have offspring find a plethora of resources available to their next generation. That doesn’t happen with this technique because the transgenic adults produce larvae which die while larvae… they’re still around and competing for resources with wild type larvae and edging some of them out.

Long time readers should notice something strange about this, I’m saying that allowing the insect to live is beneficial… especially for insects like the Screwworm and Medfly where it’s the larvae which are the most destructive. It is admittedly a bit counter intuitive, but this reduces the number of viable adults in the next generation by depriving this generation of resources.

Of course, remember that not all pests are always pests. Insects can feed on vastly different food sources between larvae and adults and one stage can be completely innocous. Mosquitoes are great examples of this, because it is only the adult female which transmits disease. The larvae eat bits of goo which build up in the small pools where they live and the adult males feed on nectar and usually don’t bother anybody. Because the medfly and screwworm cause problems as larvae we could argue about whether benefit number two is a huge benefit… but in the case of the mosquito this is fairly clear cut.

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Joe Ballenger is an entomologist who works in the biotech industry as a contractor. In his spare time, he helps answer questions about bugs at Ask an Entomologist.https://askentomologists.wordpress.com/